Misbehaving pulsar's sudden slow-down may teach us how they tick

Not all the universe’s clocks tick reliably. After decades of stability, a fast-rotating baby pulsar called B0540-69 recently slammed on its brakes. It’s the brightest and youngest one we’ve ever seen shift its identity this way, and its unpredictable behaviour will help astronomers figure out why pulsars shine in the first place, what makes them stable and what shakes them up.

Pulsars are the dense neutron stars left after supernovas. They blast beams of radiation from their magnetic poles, which sweep past Earth like the rays from a lighthouse as they rotate.

Advertisement

Many pulsars spin and gradually wind down at such reliable rates that scientists consider them “cosmic clocks”, using them to test theories like general relativity and search for gravitational waves.

Astronomers calculate a pulsar’s age based on how its spin compares to its braking. In their young years, pulsars rotate much faster and also slow down faster than their mature counterparts, before they lose energy and enter maturity. B0540-69 clocks in at just 1700 years old.

But some pulsars’ timekeeping is out of joint. Intermittent pulsars flip-flop between two states. In the “off” state, they hardly emit radio waves, and they slow down gradually. But when “on”, they beam bright and brake harder. Previously, astronomers had only seen the spin switch occur in older, dimmer pulsars that flip back and forth regularly. That made B0540-69’s sudden slow-down a surprise.

Not so stable spinner

For 27 years after its discovery in 1984, B0540-69 slowed at the same steady rate. But astronomers announced this week that in December 2011, according to observations from the the Swift and RXTE X-ray telescopes, it suddenly started slowing down 36 per cent faster. Frank Marshall of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, had watched the pulsar’s X-rays for more than 12 years and thought of it as a stable spinner. Then he saw it change radically in a matter of weeks.

The pulsar’s magnetic field might be to blame for its misbehaviour. When they are on, intermittent pulsars’ magnetic fields fill with plasma – a sea of charged particles, such as electrons and protons – and this generates radio waves. When pulsars switch off, that means the magnetic field lines have changed and let plasma leak out. Further observations of B0540-69 can help confirm that explanation, or point to another one.

Categorising B0540-69 as “intermittent” will require further observations of its radio waves. But understanding how this bright young thing fits into the puzzle will help astronomers answer fundamental questions about pulsars, like what’s inside them and why they shine at all.

“Sometimes it is the strangest behaving sources that teach us the most,” says Victoria Kaspi of McGill University in Canada. The naughty ones teach us how the nice ones tick.